Semiconductor devices have found a myriad of applications in every electrical circuit produced for consumer, commercial, industrial and military uses. Semiconductor devices, like many other electrical components, are susceptible to damage or destruction when exposed to excessive currents and voltages. When the need arises, circuits can be designed to provide protection to the devices that may encounter the overcurrents and overvoltages. Generally, as the sophistication of the protection circuits increase, the expense and reliability are adversely affected.
Depending on the magnitude of current that may have to be accommodated during an overcurrent situation, different protection devices can by employed. If small currents, in the range of 0.5 amp or less, are involved, then conventional bipolar transistor or field effect transistor (FET) circuits can be utilized to shunt the overcurrent away from the circuit to be protected. For larger currents, power thyristors can be used as the device for shunting the overcurrent away from the circuit to be protected. Thyristor devices, such as silicon controlled rectifiers (SCRs), can be used for carrying DC currents, and triacs can be used for carrying AC currents. Gated thyristors constitute a family of devices that have at least three terminals, which are sometimes called a cathode terminal, an anode terminal, and a gate terminal. The gate terminal is used for driving the device into conduction when appropriately energized. Some thyristor devices, such SIDACtor® overvoltage protection devices, employ only an anode and a cathode terminal. These two-terminal devices are generally used to protect circuits from overvoltages. A family of overvoltage protection SIDACtor® devices providing overvoltage protection is available from Teccor Electronics, Irving, Tex.
Thyristors are often used as the device to carry the overcurrent, because the associated sensing circuit are of a very fundamental nature. For example, the anode and cathode terminals can be connected between ground and the conductor carrying circuit current. The gate terminal can be connected to a resistor circuit which senses the circuit current. The value of the resistor generally sets the threshold of current carried by the circuit before the thyristor is gated into conduction to shunt the overcurrent from the conductor to ground.
The prior art is replete with teachings of the use of thyristors for protecting circuits from overvoltages and overcurrents. U.S. Pat. No. 5,625,519 discloses the use of a triac-type of thyristor in a shunt switching circuit to shunt the overcurrent around the load. U.S. Pat. No. 5,657,195 discloses the use of a gated turn-off (GTO) thyristor in series with the load. When the GTO thyristor is placed in a blocking state, the thyristor prevents current from reaching the load. U.S. Pat. No. 5,652,575 teaches the use of a triac connected across two lines to prevent an overvoltage on the lines from affecting the other circuits.
Thyristor devices, especially SCRs and triacs, are constructed as multiple-layer regenerative semiconductor devices. Such type of devices can be driven into an initial state of conduction, and if the external circuits connected thereto allow the appropriate voltage conditions across the thyristor, such devices can be driven into a latched state where the device is in full conduction and the voltage across the anode and cathode is on the order of a volt or so. When a thyristor is driven into the latched state, the device remains in full conduction even in the absence of the gating signal. The thyristor devices remain in the latched state until the anode-cathode current falls below a specified holding current (Ih). The latched state of a gated thyristor depends not only on the extent of the gating signal, but also on the magnitude of voltage between the anode and gate. This latter parameter is generally of little or no concern, as the load to be protected usually has a sufficient resistance or impedance. However, in those circuit configurations where the load has very little impedance, the gated thyristor may not be able to be latched, even though a large overcurrent exists and the gate-cathode junction of the device is forward biased. Should this adverse situation occur, the overcurrent will continue to pass through the load and the gate-cathode junction of the thyristor, but the thyristor will not be in a very low impedance latched state to otherwise shunt the overcurrent around the load. Hence, the use of gated thyristors to provide overcurrent protection to low impedance loads has not been practical.